How to use the interactive graph / analyse tool

The interactive graph on this site uses the
analyse tool to create the data.
I'll describe here how to use the interactive graph - it should be
fairly clear from this and the usage output of analyse how this
works if you're using the tool directly

Sources

The first thing you have to do is to choose a raw data source.
This site automatically checks for new data at the public websites
of the various organisations listed in the credits
page. So you should always have the latest and best data to
hand.

Processing steps

After choosing your raw data source, you can then choose a number
of processing steps to apply to the data to create the graph. If
you don't choose any, you just get the raw data itself.

The steps can be applied in any order, and can even be repeated
if you're feeling really adventurous. I've limited them to 10
for the interactive graph to stop the table getting too big, but
in the analyse tool you can have as many as you like
(if you really need more than 10 steps I might suggest you should
be running it locally!)

Some steps take a numeric value which defines what they do - what
the value does is defined below. Others don't take a value and
you can just leave it blank.

1: Data selection

These are the easiest, and just select the time window you
want to graph

Processing step

Value

Function

From

Year

Selects data from the given year onwards

To

Year

Selects data up to the given year (not including it)

Last

Samples

Selects the last number of samples given

Every

Samples

Selects samples at the interval given (usually 12)

If you want to select exact months, you have to use the
decimal year, so June 1944 is 1944.42 (June = month 6, counting from 1; month 5 counting from zero; 5/12 = 0.42).

Examples:

Note that since the graphs are auto-scaling, trend lines
by themselves will always be the same shape! It's the vertical
scale that matters.

5: Frequency-domain (Fourier) analysis

These are the most complex and most powerful of the processing steps. Fourier analysis works on the principle that any waveform can be decomposed into a combination of simple sine waves (here is Wikipedia's page on Fourier analysis).

Note: When using these processes keep in mind whether you are in the
frequency domain or time (normal) domain, otherwise you can get some
very strange results indeed!

Processing step

Value

Function

Fourier

-

Converts the data into the frequency domain. The first resulting sample is the DC offset, the second the 1st harmonic, etc.Technical note: this a complex DFT of a real dataset, so the result is symmetric around N/2

Inverse-Fourier

-

Converts the data back to the time domain again.Technical note: Complex IDFT, only real parts kept in output.

Low-Pass

Harmonic number

Removes all frequencies from the given harmonic number and above. Must be used in frequency domain.Technical note: Zeros samples from n to (N-1-n)

High-Pass

Harmonic number

Removes all frequencies up to the given harmonic number. Must be used in frequency domain.Technical note: Zeros samples from 0..n-1 and (N-n)..(N-1)

Window

-

Applies a Hann window to the data before Fourier analysis. This shapes the data so it produces a more accurate frequency spectrum, but don't rely on the shape of the output, particularly at each end.Must be used in time domain.

Magnitude

-

Takes the absolute magnitude of the complex Fourier transform. You must use this to display frequency spectra, otherwise the complex component will be lost.

Examples:

Tips & Tricks

Multiple series

You can plot multiple series at once by clicking "Add Series" and filling in another source/steps form for that series. Click "Delete Series" to remove one if you have more than one.

When there are multiple series the axes of the graph are set to the
outer range of each - hence you may need to apply 'scale' and 'offset' factors to one or more of the series to get a usable comparison.

Listen to the data!

There is an experimental feature where you can generate an audio
file from the graph. The data samples are normalised,
then used as 44.1Khz
samples and then looped 128 times and turned into an MP2 file (not MP3 because of patent issues).

Thanks to Tony Edwards for first suggesting using one of the best
pattern-recognition devices for repeating signals available!

Linking to WoodForTrees.org

The URLs used in the interactive graph generator are designed
to be easy to link to, and indicate what is happening on the
graph. If you want to link to a graph on WoodForTrees.org,
please copy the URL from the interactive page itself -
this will allow others to play with parameters themselves.

The graph page is designed to work well in blog 'SnapShots', as
it has the plot in the top-left-hand corner. However, if you
need to link to the graph itself, simply replace 'plot' with
'graph' in the URL. Similarly to get the raw data, replace 'plot'
with 'data'.